Method for producing isoprene and formaldehyde from 4.4-dimethylmetadioxane



Oct. 23, 1962 M. HELLlN ETAL 3,060,23 METHOD FOR PRODUCING ISOPRENE ANDFORMALDEHYDE FROM 4.4-DIMETHYLMETADIOXANE Filed Sept. 28, 1959 4Sheets-Sheet 1 as FIG.2 F|G.3

INVENTOR5 MICHEL HELL/N FERN/1ND COUSSE/M/VT DAN/EL LUMBROSO JEAN-PIERREswam/0 BY MARCEL ALEXANDAE m ATTORNEYS Oct. 23, 1962 M. HELLIN ETAL3,060,239

METHOD FOR PRODUCING ISOPRENE AND FORMALDEHYDF.

FROM 4.4-DIMETHYLMETADIOXANE Filed Sept. 28, 1959 4 Sheets-Sheet 2INVENTORS MICHEL HELL/N FERNAND COUSSEMANT DAN/EL LUMBROSO JEAN-PIERRESERVAUD ATTORNEYfi Oct. 23, 1962 M. HELLIN ETAL 3,060,239

METHOD FOR PRODUCING ISOPRENE AND FORMALDEI-IYDE FROM 44-DIMETHYLMETADIOXANE Filed Sept. 28, 1959 4 Sheets-Sheet s INVENTORSM/CHEL HELL/N FERNA/VD 6OU5$EMANT DAN/EL LUMBRQSO BY JEAN-PIERRE SERWUDATTORNEY$ Oct; 23, 1962 M. HELLIN ETAL 3,060,239

METHOD FOR PRODUCING ISOPRENE AND FORMALDEHYDE Filed Sept. 28, 1959 FROM4. 4-DIMETHYLMETADIOXANE 4 Sheets-Sheet 4 INVENTORf} M/CHEL HELLINFERN/4ND COUSSEMANT D4N/EL LUMBROSO dEA/V-P/ERRE EERVAUD BY MARCELALEXANDRE ATTORNEYS United States Patent 3,060,239 METHUD F01! PRODUCINGISUPRENE AND FORMALDEHYDE FROM 4.4 -DIMETHYL- METADIOXANE Michel Hellin,Rueil Malrnaison, Fernand Coussernant,

Paris, Daniel Lurnbroso, Le Vesinet, Jean-Pierre Servaud, Paris, andMarcel Alexandre, Chatou, France, assignors to Institut Francais duPetrole des Carburants et Lubrifiants, Paris, France Filed Sept. 28,1959, Ser. No. 842,839 Claims priority, application France Sept. 29,1958 7 Claims. (Cl. 260--606) The present invention relates to a methodfor producing isoprene. More in particular, the present inventionrelates to a method for producing isoprene by catalytic decomposition of4.4-dimethylmetadioxane.

Until very shortly 4.4-dimethylmetadioxane was a very costly product andit was, therefore, not used for the production of isoprene on anindustrial scale. As of recently, however, a method has been found toproduce 4.4-dimethylmetadioxane in a much more economical manner. Thismethod is disclosed in our co-pending applications Serial Number 722,848filed March 3, 1958 and now Patent No. 2,962,507, Serial Number 797,275filed March 4, 1959 and now Patent No. 2,997,480 and Serial Number830,033 filed July 28, 1959 and now abandoned.

Since this method has been found, the production of isoprene on thebasis of 4.4-dimethylmetadioxane has become quite interesting and ofeconomic importance. In order to render such a process economical andprofitable it is, however, necessary to obtain a good yield of the finalproduct. With other words, the final yield of isoprene as well as offormaldehyde, which latter is also obtained, is comparatively high withrespect to the basic product, that is the 4.4-dimethylmetadioxane. Thiseifect can only be achieved if it is possible to limit the secondaryreactions whereby 4.4-dimethylrnetadioxane is converted to isobutane,3-methylbutane-1.3-diol and heavier products; furthermore, it isnecessary to limit the resinification of the isoprene and theformaldehyde, which results in the deposition of carbon on the catalyst,which latter has to be regenerated very frequently.

The known processes for producing isoprene from 4.4- dimethylmetadioxanedo not meet these requirements. The industrial application of the knownmethods is far from profitable since the selectivity of these knownprocesses is very poor. By selectivity we understand that quality of thereaction process which makes it possible to obtain a good yield ofisoprene with respect to the quantity of converted4.4-dimethylmetadioxane.

In the known methods causing a reaction of 4.4-dimethylmetadioxane inthe liquid phase, the selectivity of the reaction is rather poor. Inaddition, the process is difficult to carry out since no satisfactorysolution has been found of the problem of how to assure a satisfactorycontact between the catalyst and the 4.4-dimethylmetadioxane and, at thesame time, to remove rapidly the products of the reaction in order toavoid their de terioration and decomposition.

It has already been proposed to efiect the reaction in the vapor phase.However, in these methods the catalysts are poor and inefficient.Particularly the selectivity of the catalysts is insufficient toaccomplish the optimal rate of transformation and to assure aneconomical and profitable industrial production of the isoprene.

It is, therefore, an object of the present invention to provide a methodfor producing isoprene by catalytic decomposition of4.4-din1ethylmetadioxane which is much more economical than any of theknown processes and which makes it possible to produce isoprene from4.4- dimethylmetadioxane efiiciently and profitably on an industrialscale.

It is another object of the present invention to provide a method forproducing isoprene by catalytic decomposition of 4.4-dimethylmetadioxanewhich has a high con version rate, a high selectivity and a high yieldof isoprene and formaldehyde.

It is a further object of the present invention to provide a method forproducing isoprene by catalytic decomposition of4.4-dimethylrnetadioxane, in which high losses of formaldehyde andisoprene are avoided and the resinification and decomposition ofisoprene and formaldehyde are greatly reduced.

It is still another object of the present invention to provide a methodfor producing isoprene by catalytic decomposition of4.4-dirnethylmetadioxane, as well as a catalyst for this method, whichcatalyst has a great mechanical and thermic resistance, can be easilyregenerated and has a very long service life.

These objects as well as further objects and advantages of the inventionwhich will become apparent as the detailed description thereof proceeds,are achieved by the method of the present invention whereby isoprene canbe profitably produced on an industrial scale by the catalyticdecomposition of 4.4-dimethylmetadioxane according to the followingreaction:

0E3 CHPCHZ CH;

O CHF-OH=C/H2 HCHO H2O CH3 O-CH2 According to the method of the presentinvention the 4.4-dimethylmetadioxane is passed in the vapor phase overa catalytic agent consisting of a silica having a small specificsurface, which silica has previously been impregnated with apredetermined quantity of phosphoric acid. In carrying out this processa number of further conditions have to be observed which will bedescribed further below. The method of the invention comprises thefollowing basic steps: A catalyst is prepared by impregnating silicahaving a small specific surface with a predetermined quantity ofphosphoric acid and in a manner described in further detail below.

4.4-dimethylmetadioxane is then passed in the vapor phase over thiscatalytic agent, preferably after having been diluted with inert gasesor vapors, such as parafiinic or naphthenic hydrocarbons or ahydrocarbon mixture as obtained by the reaction process for making4.4-dimethylmetadioxane, described in the co-pending applications,supra, or with steam.

The gaseous mixture is then tapped from the reaction vessel and isfractionated by fractional distillation or by selective extraction witha solvent such as a paraflinic or naphthenic hydrocarbon or a mixture ofboth.

The invention also provides for a method for regenerating the catalyticagent after it has been used for some time, in order to regain itsinitial efiiciency.

Describing now the invention in greater detail and turning to the firstbasic step, a catalyst is prepared which is composed of silica having asmall specific surface and which is then impregnated with phosphoricacid. By small specific surface we wish to be understood a specificsurface which is not greater than m. per gram and which is preferablyless than 20 m per gram. The specific surface can be measured forexample with the apparatus described by Brunauer, Emmett et Teller, J.Am. Chem. Soc. 60, 309, 1938.

It has been found that the silica support materials for the catalyst,which best meet this requirement of a small specific surface, arequartz, silicious sand or sandstone, particularly in form of grains oragglomerated microparticles. We found that these silicious materials areparticularly advantageous as a catalyst support, because of theirexcellent physical properties and their remarkable catalytic properties,if processed according to the invention and impregnated with phosphoricacid. For that reason quartz, silicious sand or sandstone are preferablyused as a support material for the catalyst. Other forms of silica areless advantageous though they still give a good yield and a satisfactoryselectivity of isoprene.

This result is entirely unexpected and surprising. Quartz, silicioussand and sandstone do not have any catalytic activity per se furtheringthe decomposition of 4.4- dimethylmetadioxane. However when treated inthe manner to be presently described and impregnated with phosphoricacid they provide for a far better selectivity of conversion of4.4-dimethylmetadioxane in the vapor phase than that obtained by usingphosphoric acid in the liquid phase without support or by using anactive catalyst support such as the silicoalumina.

The high unsatisfactory yield of isoprene obtained with silicoalumina asa catalyst is illustrated by the following example:

A mixture of 900 grams of 4.4-dimethylmetadioxane and 910 grams of wateris passed through a catalytic bed composed of 304 grams of syntheticsilicoalumina. The speed with which the 4.4-dimethylmetadioxane and thewater are injected into the reaction vessel is 0.12 liter/hour. As aresult of the reaction there are obtained only 81.2 grams of isoprene,150 grams of isobutene, 32.8 grams of pentenes, 58 grams of formaldehydeand 305 grams of non-converted 4.4-di-methyl-metadioxane. The depositson the catalyst are as high as 120 grams. The Weight of the thusobtained isoprene represents a molar yield of only 23.3% with respect tothe converted 4.4-dimethylmetadioxane.

This example shows that the use of silicoalumina does not assure aselective decomposition of 4.4-dimethylmetadioxane so as to obtainisoprene; in addition, the yield of formaldehyde is very poor.

According to the invention the catalyst is prepared by using the silicasupport material, in the aforementioned specific surface ranges, as asupport material which is then impregnated with a predetermined relativeamount of phosphoric acid. The phosphoric acid content of the catalystcan be expressed, for example, in a certain per centage by weight ofphosphoric acid with reference to the weight of the impregnated supportmaterial.

The impregnation must be so controlled that this percentage is Withindetermined limits, because both a percentage which is too small and apercentage which is too high is disadvantageous and does not permit toobtain most profitable results of the method of the invention. It thepercentage of the phosphoric acid is too small the conversion rates of4.4-dimethylmetadioxane and of isoprene are very poor and the process isnot profitable. On the other hand if the support contains too much acid,the catalyst favours secondary reactions and carbonization effectswithout raising the conversion rate sufficiently to compensate for thesecondary reaction losses. We have found that optimal results areobtained if the silica is impregnated with an amount of phosphoric acidin the range of from 0.3 to 5% by weight. Preferably the acid content ofthe silica support is from 1 to 2.5% by weight. It is to be understoodthat these percentages relate to the total end weight of the impregnatedsupport material after drying.

The silica support can be used in the form of small pills or grains. Itis, for example, possible to use grains having a diameter in. the rangefrom 0.5 to 2.5 millimeters.

The silica support is impregnated with phosphoric acids, for example bypreparing an aqueous solution of phosphoric acid having a concentrationdepending on the desired acid content of the catalyst. The supportmaterial is then immersed in this aqueous solution which can be done atnormal atmospheric pressure or in vacuum. Thereafter the thusimpregnated material is dried in a dryer at a temperature of from 120 to700 C. for a period of e.g. 2 to 20 hours. At a temperature of about 280C. the drying takes e.g. about 10 hours. The duration of the dryingprocess is inversely related to the degree of temperature used.

Any other way of bringing the support in contact with the impregnatedagent may be used, as for example passing liquid impregnating agentthrough the support material or by contacting the support withimpregnating agent in a finely dispersed state of the latter.

After having thus prepared a catalyst the reaction process can becarried out with the use of this catalyst. The 4.4-dimethylmetadioxaneis passed over this catalyst at a temperature, a pressure, and a spatialspeed which will next be explained separately in detail.

The temperatures at which the 4.4-dimethylmetadioxane is passed over thecatalyst at a given spatial speed should be higher than 200 C. atatmospheric pressure. According to the invention, the preferredtemperature range is between 250 and 280 C. It is absolutely necessaryto avoid temperatures above 300' C. because at higher temperatures theformaldehyde will decompose. We have found that at a temperature of, forexample, 270 C., which is in the aforementioned temperature range, theyield of formaldehyde is of the theoretical yield whereas it drops toless than 50% if the temperature rises above 300 C., ceteris paribus.

Turning now to the pressure during the reaction, it may be advantageousto operate at a reduced pressure in view of reducing deposits of carbonon the catalyst, however, in practice it is most expedient to operateunder normal atmospheric pressure. It may be also interesting in somecases to operate at higher pressures, for example up to about 5kilograms per cm. since in that case higher spatial speeds can beapplied While using an apparatus of the same volume.

The 4.4-dimethylmetadioxane is passed in the vapor phase through thecatalytic bed at the spatial speed which is determined according to thedesired rate of conversion. The latter can be increased by diminishingcorrespondingly the spatial speed of the 4.4-dirnethylmetadioxane.

As a general rule it is preferable to limit the transformation rate toxavalue which is less than 90% and preferably in the range of 60% of the4.4-dimethylmetadioxane passed over the catalyst, in order to reduce theresinification of isoprene and of formaldehyde. This resinification isparticularly disadvantageous because it not only reduces the yield ofisoprene and formaldehyde but at the same time lowers the activity ofthe catalyst. It is therefore justified to voluntarily limit theconversion rate, particularly in view of the fact that this does notprejudice the optimal yield as the process is a continuous one, and thefinal yields of isoprene and formaldehyde with respect to the4.4-dimethylmetadioxane can still be very high by refeeding the4.4-dimethylmetadioxane which has not been converted into the reactionvessel.

The rate of conversion can be limited, for example, by lowering thetemperature in the reaction vessel, or by increasing the spatial speedof the 4.4-dimethylrnetadioxane in the reaction vessel, thereby reducingthe time during which the mixture is in contact with the catalyst.

It is also of advantage to limit the spatial speed of the4.4-dimethylmetadioxane through the catalyst to such a value thatsubstantially phosphoric acid is taken along. It has been found as ageneral rule that a spatial speed of the 4.4-dimethylmetadioxane withrespect to the catalyst in the range of from 0.2 to 3 liters per hourand per liter of catalyst is preferred and results in a conversion ratein the range from 30 to 80%.

We have also found that the resinification of the isoprene can befurther reduced by diluting the 4.4-dimethyl metadioxane with inertgases or vapors, such as, for example, nitrogen, steam or parafiinic ornaphthenic hydrocarbons or mixtures thereof, which can be easilyseparated from the isoprene by fractional distillation such as, forexample, the hydrocarbon mixtures obtained from the units producing4.4-dimethylmetadioxane from a cracking C cut containing isobutene.

The rate of conversion as conditioned on a particular catalyst, a giventemperature and a given spatial speed of the 4.4-dimethylmetadioxane isnot substantially changed by the addition of the aforementioned inertvapors or gases with which the 4.4-dimethylmetadioxane is diluted.

On the other hand, an excessive dilution should be avoided as this wouldabsorb a substantial amount of heat, thereby rendering the process muchmore expensive. As a practical compromise we have found that if, forexample, steam is used as a diluting agent, the molar proportion of thewater with respect to the total feed charge is in the range of from30-95% After the 4.4-dimethylmetadioxane has thus been passed throughthe catalyst after the reaction has taken place converting a givenpercentage of the 4.4-dirnethylmetadioxane, the gaseous mixture istapped from the reaction vessel and is separated. This separation can beeffected either by fractional distillation or by selective extraction byadding a solvent.

By this separation, isobutene, isoprene and the aqueous solution offormaldehyde as well as high molecular condensation products areobtained. In addition, a small portion of 4.4-dimethylmetadioxane isobtained which has not reacted and which is reintroduced into thereaction vessel.

Turning first to the separation by fractional distillation, this can becarried out advantageously in the following manner: The vapors tappedfrom the reaction vessel are condensed and the liquid obtained is thenfed into a separator. The upper layer of the liquid constitutes theorganic phase and is fed into a distillation unit, such as afractionation column, where it is distilled and the various products areseparately collected, which are isoprene, traces of isobutene,4.4-dimethylmetadioxane which has not reacted and which is re-introducedinto the reaction vessel for further processing, as well as a smallquantity of residual substances having a higher molecular weight thanisoprene. The aqueous phase forming the lower portion of the liquid inthe separator is introduced into a second fractionation column at thehead of which there is obtained an azeotropic mixture of4.4-dimethylmetadioxane and water which is then condensed and wherefromthere is separated by simple decantation the major por tion of the4.4-dimethylmetadioxane which is then re-fed into the reaction vessel,whereas the aqueous portion, which still contains a small amount of4.4-dimethylmetadioxane in solution, is re-fed into the fractionationcolumn. At the bottom of this column there is collected a dilutedaqueous solution of formaldehyde which can be concentrated, for example,by concentration under superatmospheric pressure and which is thenre-fed into unit producing 4.4-dimethylmetadioxane, from isobutene,working according to the process described in the copendingapplications, supra.

Instead of separating the isoprene and the isobutene by fractionaldistillation it is also possible to proceed by selective extraction.This is done by adding inert solvents which can be easily separated bydistillation. As a solvent it has been found to be of advantage to usehydrocarbons or mixtures thereof, particularly paraffinic or naphthenichydrocarbons or mixtures thereof having at least four carbon atoms andwhich are easily separable from the isoprene by distillation. Thegaseous mixture tapped from the reaction vessel is extracted by addingsuch as a solvent, whereby an organic solution is obtained. The varioussubstances obtained from this distillation are consisting of isoprene,traces of isobutene, the solvent of 4.4- dimethylmetadioxane, whichlatter is re-fed into the reaction vessel.

The aqueous phase containing the formaldehyde may be concentrated underpressure and can then be used as one of the basic materials in theprocess for the manufacture of the 4.4-dimethylmetadioxane described inthe co-periding applications, supra.

The use of the catalyst according to the method of the invention andgene-rally the process of the invention results in a highly profitableproduction. By using the method of the invention, molecular yields areobtained which are in the range of about isoprene and formaldehyde withrespect to the converted 4.4-dimethylmetadioxane, whereas the yield ofisobutene does not exceed S The catalyst prepared as described above andused for converting 4.4-dimethylmetadi0xane can be used for aconsiderable time of operation and thus is highly economical.

In addition, it can be re-generated after its catalytic effect has shownsome deterioration and can then be reused in the process of theinvention. The re-generation of the catalyst is effected byreimpregnating the same with phosphoric acid in the same manner as theinitial impregnation.

This re-impregnation can be repeated for several times, although notindefinitely, because the reimpregnation does not remove the deposits onthe catalysts, which, in the long run would greatly reduce theircatalytic activity. According to the invention it is, there-fore,suggested to eliminate these deposits periodically after a number ofsuccessive re-impregnations, for example by burning the catalyst in airor oxygen at a temperature in the range of from 400 to 500 C. forseveral hours. After having done this, the catalyst is re-impregnatedwith phosphoric acid and can be used for a considerable time and can berepeatedly reimpregnated.

Although excellent results are thus obtained by using the methoddescribed heretofore, we have found that by taking additional steps andprecautions the results ob tained by the invention can be furtherimproved by the preferred embodiment of the method of the presentinvent-ion which is preferably carried out with the apparatus describedbelow. 7

According to a preferred embodiment of the invention the catalyticparticles of the afore-described type are displaced relative to oneanother during the process of reaction. This displacement can beeffected either periodically or permanently.

If the catalytic particles are displaced periodically rather thanconstantly the intervals between each period of relative displacement ofthe particles should not be too great. We have found that an interval ofabout?! to 5 hours is the maximal allowable interval between twosuccessive relative displacements of particles. It is, however, verywell possible to make this interval shorter.

Preferably, this displacement is effected continuously since it resultsin a most even catalytic effect. If the displacement is effectedperiodically rather than permanently it must be more efficient. Thelonger the inter-- vals between two successive displacements, the morevigorously and efiiciently each operation of displacement must becarried out.

The displacement can be effected mechanically. According to the presentinvention it can also be effected by passing a stream of gas containingthe 4.4-dimethylmetadioxane in the vapor phase through the catalyticagent, thereby bringing the substance to be converted into contact withthe catalyst and at the same time displacing particles of the latter,thereby attaining a more efiicient, speedier and more productivereaction. The gas stream can be passed through the catalyst at variousspeeds. If the speed is comparatively moderate the catalyst particlesare simply displaced with respect to each other so as to obtain What maybe called an expanding bed of catalytic material. If the gas stream ispassed through the catalyst at a comparatively high speed the catalyticparticles are so quickly taken along by the gas stream that they arevirtually suspended therein, thereby obtaining what may be called afluid bed of the catalyst.

The displacement becomes more effective in direct proportion to anincrease of the speed of the gas stream, up to and including the limitwhere the high Speed of the gas stream keeps the grains of the catalystin suspension.

We have found that a speed resulting in an expanding bed is generallysufficient :for obtaining good results and it is not absolutelynecessary to increase the speed up to the point where the catalyst formsa fluid bed.

111 order to efi'ect this last-mentioned method of displacement, that isconstituting a fluid bed of the catalytic particles by passing the gasstream of the reactants therethrough, the particles of the catalyticagent must be significantly smaller than in case the catalytic particlesare used in the form of a fixed bed. We have found that the particlesmust have a size within very strict limits which are approximately inthe range from 20 microns to l millimeter, and preferably between 50 and500 microns. These ranges are not to be considered as exclusive and aresubject to variations depending primarily on the speed of the gasstream. Preferably, the highest speeds are associated with the largestparticles of a catalyst, and vice versa.

According to a further embodiment of the method of the invention, heatis supplied to the catalyst and to the gas in the interior of thereaction vessel.

The reaction converting the 4.4-dimethylmetadioxane is highlyendothermic and for that reason heat must be supplied. It would beobvious to supply this heat by heating the gas prior to its introductioninto the re action vessel, thereby providing for the necessary heatrequired by the reaction therein. However, we have found that it wouldbe greatly disadvantageous to do that in as much as it would severelylimit the rate of conversion and would result in a very poor yield ofisoprene and formaldehyde. As a matter of fact, the gases fed into thereaction vessel have a comparatively low specific heat and therefore itis necessary to overheat them very strongly in order to provide for thenecessary heat required by the reaction. Since parasitic reactionsdevelop very rapidly starting from a certain temperature which is in therange of about 300 (3., a notable reduction of the yield of isoprene andformaldehyde would be the result of such a preliminary overheating.According to the present invention it has been found to be advantageousto supply the heat requirements in the interior of the reaction vesselitself and to maintain the catalytic material within the reaction vesselat a substantially homogeneous temperature. Furthermore, the heat isvery evenly distributed in order to prevent local over-heating of anyportion of the catalytic mass. This can be done in the most efficientmanner by disposing a large heat-exchanging surface within the interiorof the catalytic mass.

. e According to still a further embodiment of the method of theinvention the gases leaving the reaction vessel are rapidly cooled. Wehave found that such a rapid and eflicient cooling of the gases furtherincreases the yield of the reaction. In addition, such a process avoidsthe formation of solid substances which could disturb the circulation ofthe products of the reaction, for example by blocking the sealing means,polluting the heat ex changers, etc.

The gases must be chilled as rapidly as possible in '8 order to lowertheir temperature from the reaction temperature, which latter is up toabout 300 C. down to a temperature in the range from 20 to C. in theshortest possible time.

The above-described method of regenerating the catalyst is also furtherimproved by the following modifications: The regeneration can beeflfected either periodically or preferably continuously.

It can be done continuously, for example, where the catalyst forms afixed bed in which case the catalytic mass is regenerated in itsentirety, by burning the deposits and/or reimpregnating the same withthe phosphoric acid. The regenerating of the entire catalyst does in noway prejudice the continuous operation of the reaction process since tworeaction vessels can be provided, one of which is in operation whereasthe other is taken out of operation for regenerating the catalysttherein.

The same periodical regeneration can be effected Where the catalystforms a mobile or fluid bed. According to a preferred embodiment of themethod of the invention this is, however, done continuously. Accordingto this part of the method of the invention the circulation of thecatalyst is used for effecting a continuous burning of the depositsthereon and a subsequent reimpregnation in a unit separate from thereaction vessel. This method is particularly useful where the catalystforms a fluid bed since a very smooth and entirely continuous operationis thus obtained.

The afore-mentioned steps can be taken both for removing the deposits onthe catalyst and for impregnating the catalyst with phosphoric acid,although the first-mentioned operation does not have to be carried outas frequently as the reimpregnatiou but only once for a number ofreimpregnations, which number is in the order of about two to ten.

The burning of the deposits is preferably carried out by an oxygencurrent or a gas mixture containing oxygen as, for example, air, whichis heated to a temperature in the range from 300 to 600 'C., the highesttemperature being preferably used with gas mixtures having the lowestoxygen content and vice versa.

As has been described further above, the reimpregnation of the catalystwith phosphoric acid can be done by immersing the latter into a solutionof phosphoric acid having a concentration, for example, from 10 to byweight, and then drying the catalyst in order to remove the watercontent. However, according to a preferred embodiment of the method ofthe invention, very fine droplets of an aqueous solution of phosphoricacid of suitable concentration, for example, in the range from 10 to 85%by weight, are passed through the catalytic mass of material, whichlatter is preferably in the form of a fluid mass.

It is absolutely necessary to effect the reimpregnation of the catalystin the absence of the reactants and the reaction product. Also, a directinjection of the phosphoric acid into the reactants and products of thereaction must be carefully avoided. It is, however, possible to effectthe reimpregnation into a unit connected with the reaction vessel. Inthat case and according to the preferred method of the invention a partof the steam to be introduced into the reaction vessel is used formaintaining that fraction of the catalytic particles which has to bereimpregnated into suspension. The phosphoric acid having aconcentration in the aforementioned range is then injected either intothe fluid catalyst itself or into the steam current for maintaining thecatalyst in suspension. The process of suspending the catalyticparticles in the form of a fluid bed is equivalent to an excellentdisplacement and thereby a particularly homogeneous penetration of theacid throughout the catalytic mass is obtained. In addition, thephosphoric vapor is only in contact with the steam and the catalyst, butnot with the reactants, and the charging of the reaction vessel with 94.4-dimethylmetadioxane and additional steam can be e-f fected in theabsence of any vapors of phosphoric acid.

It the regeneration unit is thus connected with the reaction vessel itis advantageous to carry out the regeneration at the same temperaturewhich is used for the reaction, as this procedure will avoid any loss ofheat in the entire system.

The aforementioned steps in the preferred embodiment of the method ofthe invention are highly advantageous and result in a process of a greatprofitability and efficiency.

We have found that a thorough displacement of the catalytic particlesrelative to one another has, in some instances, doubled the rate ofconversion and thereby makes it possible to substantially increase theproduction of isoprene as compared with that obtained by the describedembodiment of the method of the invention without displacement. Inaddition, the activity of the catalyst is greatly prolonged comparedwith the activity in the method of the invention in which the catalystforms a fixed bed.

These results are entirely unexpected because heretofore onlyregeneration methods such as calcination, reimpregnation or otherchemical, rather than pure mechanical processes brought about suchresults. The following may constitute an explanation for this: If thecatalyst forms a fixed bed, channel outlets are formed therein after acertain time of operation through which the stream of gas of thereactants is allowed to pass preferentially. These passages, which canbe caused by a displacement of the grains of the catalyst or bydeposits, put a major part of the catalyst out of action. A periodicaland preferably permanent displacement of the catalyst destroys thesepassages and thereby the entire mass of the catalyst participates in thereaction.

This explanation should be regarded as merely an attempt not limitingthe invention in any way.

The furnishing of heat to the reactants in the reaction vessel itselfrather than prior to their introduction into the latter makes itunnecessary to over-heat the charge beyond the temperature range, atwhich secondary parasitic reactions set in, which would greatly diminishthe yield of isoprene and formaldehyde.

The abrupt chilling of the gaseous products leaving the reaction vesselalso greatly increases the yield and, in addition, keeps the reactionvessel clean and prevents any disturbance of the circulation of theproducts therein.

The present invention also provides apparatus with which the method ofthe invention can be for example advantageously carried out. Theseapparatus are shown in the accompanying drawings, wherein,

FIGURE 1 illustrates, by way of an example, such an apparatus foragitating the catalytic mass, substantially comprising a horizontalcatalytic chamber having a plurality of baffie plates and a rotatingshaft with a plurality of drums rotating in the axis of the reactionvessel, and wherein an empty space is left in the upper portion of thereaction vessel in order to allow the catalytic particles to fall backinto the reaction vessel.

As shown by way of an example in FIGURE 1, the horizontally disposedreaction vessel 1 which is stationary, is provided with a plurality ofbaflle plates 2, 2a, 2b, 2c and is filled with catalyst 3 up to thelevel 4. Through catalyst 3 passes the gas stream, entering, asindicated by arrow 5, through inlet channel 6 and leaving the vessel 1through outlet channel 7 as indicated by arrow 8. The agitator iscomposed of a shaft 9 projecting from the reaction vessel 1 throughopenings 12 and 13, and bearing a plurality of drums 10; ltia, 1%, rec,19d. The shaft 9 with the drums rotates slowly, for example, in thesense of arrow 11, at a rate of to 20 revolutions per hour. Thereby thegrains constituting the catalytic mass are periodically displaced withrespect to each other.

It is also possible to provide an apparatus having a vertical reactorand wherein the catalyst is continued circulated, thus forming a mobilebed. The catalyst is tapped at the lower end of the reaction vessel andit is re-entered at its upper end. Such an apparatus is shown, forexample, in FIGURE 2. The reaction substances are fed into the catalyticchamber 21 through inlet 22, as indicated by arrow 23, and the reactionproducts leave the chamber 21 through outlet 24, as indicated by arrow25. The catalyst circulates and travels through the chamber 21downwardly in the direction of arrow 27 after having entered throughchannel 26. It then leaves through channel 28. The amount of thecatalyst leaving the chamber is controlled by known mechanical meanssuch as an Archimedes screw or movable grid which are conventional andtherefore not shown in the drawing. it is then lifted in the column 29in the direction of arrow 30 by such conventional means as, for example,a chain of buckets or a high speed gas stream. After having been thuslifted in column 29, the catalyst is refed into the reaction chamber 21through channel 26.

Another apparatus is provided for elfecting a continuous agitation bythe gas current containing the reaction substances themselves and whichis shown, for example, in FIGURE 3. The reaction vessel 31 contains thecatalyst 32. in its lower portion maintained by the distribution grid 36and has at its lowermost end a conically shaped portion 31a, ending inan inlet channel 34. In the upper portion of vessel 31 there is provideda cyclone 33 having at its upper end an outlet channel 37 and at itslower end a tube 39 projecting into the catalyst 32.

The gas stream is fed into the reaction vessel through channel 34, asindicated by arrow 35, passed through distribution grid 36 andpenetrates the catalyst 32, and then enters the cyclone 33. The gasstream then leaves the reaction vessel through channel 37, as indicatedby arrow 38. The cyclone 33 prevents particles of the catalyst to betaken along by the stream of gas by separating the fine particlestherefrom, which then fall by their proper gravity through the tube 39back to the catalytic mass 32.

The invention provides also means for heating the catalyst and the gaswithin the interior of the reaction vessel. This can be done byconventional means such as, for example, heat exchange coils which arespirally or helically shaped in the interior of the reaction vesselthrough which coils there is passed a hot liquid or vapour, orelectrical heating wires can be provided in the coils. Theseconventional means are primarily used in a horizontal reaction vessel.

Preferably, and according to the invention, an internal heat exchange iseffected by the apparatus shown, for example, in FIGURE 4, comprising aplurality of tubes disposed within the reaction vessel.

A plurality of tubes such as, for example, the tubes 43, are provided inthe reaction vessel 41, in contact with the catalyst 42 and compriseinternal tubes such as 45. The tubes 43 are projecting vertically intothe reaction vessel and are disposed parallel relative to each other. Atthe upper end of external tube 43 there is provided a return chamber 47and, above the same, a distribution chamber 46. The internal tube 45projects through return chamber 47 and opens into distribution chamber46.

The hot fluid is fed from the distribution chamber 46 downwardly, asindicated by the arrow 49, through the internal tube 45. It then risesback up to the return box 47 through the annular free space 48 formedbetween tubes 43 and 45 (arrow 44), during which latter travel itexchanges its heat to the catalytic mass 42. The reaction gas can bepassed through the catalytic mass either upwardly or downwardly.

It is also possible to provide an apparatus wherein the catalyst isdisposed in the interior of, as shown for example in FIGURE 5. Withinthe reaction vessel 51 there are disposed a plurality of tubes, such as52, having at their respective upper ends a tube builder plate 55, andat the respective lower ends a tube builder plate 54. The catalyst 53 isprovided in the tubes 52 as well as over and below the plates 54 and 55on respective heights h and h so that the volume of the catalyst isgreater than the volume of the tubes 52. The heat exchange agent mayconsist of condensing vapor or a hot liquid circulating about the tubes52 and which is supplied through channel 56, as indicated by arrow 57,and leaves through channel 58, as indicated by arrow 59. The reactiongas can be passed through the catalytic mass either upwardly ordownwardly.

This apparatus is particularly useful where the catalyst forms a mobileor a fluid bed.

Another type of apparatus is provided for getting an excellent fluid bedof the catalyst by injecting the gas at the basis of each tube andcontrolling the dosage which is injected as carefully as possible. Inthis case the catalyst cannot fill up the lower portion of the reactionvessel below the tube builder plate 54. As shown in FIGURE a, there isprovided a distribution grid 60 above which there is provided thecatalyst 53. Carefully calibrated adjusting means 61 are disposed at thebottom of each tube 52 and below grid 60, which causes a loss of chargematerial, which is substantially above that caused by the passage of thegas through the fluid catalytic mass. A layer of fluid catalyst ismaintained above the upper tube builder plate 55 which has the advantageof automatically controlling the height of the catalyst over each gridand to allow for the circulation of the catalyst particles from one tubeto the other.

The invention further provides means of abruptly and rapidly chillingthe gases leaving the reaction vessel. This chilling can be carried outwith conventional means, such as external circulation cooling means witha liquid cooling agent, which can be equipped with cooling bafiles,perforation-s, coils, tubes and other devices for increasing the coolingsurface. Preferably, however, the chilling is effected by means of theapparatus of the type, as shown schematically in FIGURE 6.

According to this cooling system, the gases and vapors leaving thereaction vessel 62 as at 62a are passed into the con-tact chamber 63where they are intimately mixed with a refrigerated liquid, preferablyconsisting of the aqueous phase of the condensed reaction substances 64.This aqueous phase is separated from the organic phase 65 by decantationand is then cooled by the cooling means 68, after the fractioncorresponding to its production in the reaction vessel 62 has beenremoved through channel 66, for which the storage container 67 is used.Thereafter, the aqueous phase is rated into the contact chamber 63 at anelevated rate.

This cooling system offers a particularly advantageous chilling methodsince it makes it possible to have a direct cooling contact between thegas and the liquid phase, instead of an indirect heat exchangetransmitted through tubes and the like, without having losses of thereaction products or the non-transformed reaction substances bydissolution in the liquid. This latter danger is avoided since theliquid phase is already saturated with reaction products and travelsthrough a closed circuit. Of course, it would be possible to use wateras a cooling liquid, however, this would then call for the additionalstep of separating the water and the products dissolved therein.

Two particular embodiments can be advantageously used in the abovecooling system. As shown in FIG- URE 7, there is provided a contactcolumn 71 of the scrubber type. At its upper end it has an outletchannel 80 and at its lower end an outlet channel 78. In the upperportion there is also introduced into the column an inlet channel 73having perforations 74. At its lowermost end there is introduced aninlet channel 75 having a distribution head 75a.

The cool aqueous phase is introduced through channel 73 and finallysprayed through perforations 74. The gas is introduced into the columnthrough channel 7 5, as indicated by arrow 76 and distributed throughthe distribution head 75a at the basis of column 71. The contact betweenthe aqueous phase and the gas is thus effected countercurrently and theaqueous phase takes along all condensed products. It then leaves thecolumn through channel 78, as indicated by arrow 79, whereas the coldnon-condensed gases leave the column through outlet 80, as indicated byarrow 80a.

According to a modification, means are provided for effecting the mixingby passing the aqueous solution through a nozzle. As shown in FIGURE 8,there is provided a channel 81 having a conical portion ending in anozzle 81a and projecting into vessel 89 through a chamber 89a. An inletchannel 83 leads to this chamber 89a, which at its lower endcommunicates with a venturi passage 85 at the uppermost neck portion85a, which is in the immediate vicinity of nozzle 81a. The lowermost endof the venturi passage communicates with chamber 86 having an outletchannel 87.

The aqueous solution is fed through channel 81, as indicated by arrow'82, and injected through nozzle 81a. The gases are drawn in throughinlet channel 83 in the direction of arrow 84 and pass into chamber 89a.The two phases come into contact at the neck portion 85a of the venturipassage 85 and are further mixed while passing into chamber 86 therebyforming a gas-liquid emulsion leaving through outlet 87, as indicated byarrow 88.

This apparatus has the advantage of creating a very great turbulence ofthe contacting phases, whereby a particularly abrupt temperature drop isproduced. In addition, the comparatively small size of this apparatusmakes it possible to have it placed very close to the reaction vessel.

For the aforedescribed catalytic regenerating process the inventionprovides an apparatus of the type shown in FIGURE 9 or of the type shownin FIGURE 10.

Turning first to FIGURE 9, a re-generation unit 90 is connected with thereaction vessel 91 by the channel 98. The reaction vessel has, in itslower portion, a distribution grid 94 and at its lowermost end, an inletchannel 93. There is provided another channel 92 forming two branches92a, 92b, the branch 92b communicating With inlet channel 93, the branch92a communicating with the regeneration unit 90. The latter has anotherinlet channel 97. A further channel 96 leads from the interior ofreaction vessel 91 to the interior of the regeneration unit 90. At itsuppermost end the reaction vessel 91 has an outlet channel 95.

Water steam is fed into the reaction vessel 91 through channels 92, 92b,93, as indicated by arrows 92d, 93a, and 4.4-dimethylmetadioxane issupplied thereto via channel 93, both of which substances pass throughdistribution grid 94 into the catalyst 99. The finely grained catalystis maintained in suspension within the reaction vessel 91 by the streamof water steam and 4.4-din1ethylmetadioxane in the vapor phase. Thegaseous products leave the reaction vessel through outlet channel 95 asindicated by arrow 95a, whereas the catalyst enters into channel 96,travels therein as indicated by arrow 96:: and then passes into theregeneration unit 90, in which latter it is suspended by the stream ofwater steam arriving through channels 92, 92a, and distribution grid94a, as indicated by arrow 920. A pre-determined quantity of dilutedphosphoric acid, as indicated by arrow 97a, is introduced into there-generating unit 90 either periodically or, preferably, continuously.The quantity of phosphoric acid is so adapted as to maintain thephosphoric acid content of the catalyst on a constant level. Thecatalyst is thus re-impregnated and while forming a fluid bed, passeswith the steam into the reaction vessel 91 through channel 98, asindicated by arrow 98a.

The detailed construction of the reaction vessel 91 has been omitted inFIGURE 9 for the sake of clarity; it can, of course, comprise allconventional elements or 13 the other described elements, andparticularly a cyclone for separating the catalytic particles from thegas stream.

Another type of apparatus for regenerating the catalyst according to theinvention is shown in FIGURE 10. The reaction vessel 100 is composed oftwo portions communicating wth each other, a reaction portion 101, anintermediate annular space 109 and a regeneration portion 102. Theregeneration unit 102 has an inlet channel 108 communicating with thebranches 107 and 107. A grid 106 is provided in the lower portion of theregeneration unit and a grid 106a is provided in the lower portion ofthe reaction unit 101. The two units are in communication through theintermediate space 109', and through channel 110. Furthermore, there areprovided two inlet channels 103 and 104, uniting to form a channel 105which is passed into the regeneration unit 102 and then leads directlybelow grid 106a in the reaction unit 101. The latter unit has an outlet111 at its uppermost end.

The 4.4-dimethylmetadioxane is fed into the reaction unit 101 throughchannels 103, 105 and distribution grid 106a (see arrows 103a, 105a) andit is supplied with water steam through channels 104, 105 and grid 106a(see arrows 104a, 105a); the two substances mix already when passingtogether through channel 105 and are evenly distributed when passingtogether through the distribution grid 106a. The amount of Water steamthus fed into the reaction unit forms only a fraction of the necessaryquantity for diluting the reacting substances. Therefore, additionalsteam, mixed with small quantities of phosphoric acid, introducedthrough channel 107, as indicated by arrow 107a, is introduced throughchannel 108 (see arrow 108a). The mixture being evenly distributedthrough grid 106 thus reaches the catalyst 99 in the regeneration unit102. It suspends the catalyst and, at the same time, re-impregnates thesame with phosphoric acid. The fluid mixture then passes into thereaction unit via the annular space 109. The catalytic particles returnto the regeneration unit through channel 110, whereas the gases leavethe reaction unit 101 through outlet 111.

The two types of apparatus shown in FIGURES 9 and 10 thus have in commonthat a part of the water steam to be used for the reaction in thereaction unit is used for making fluid a portion of the catalyst whichhas to be impregnated.

Where the catalyst forms a fluid or mobile bed, it is also possible toreimpregnate the catalyst with phosphoric acid outside of the reactionzone and in the absence of the reaction substances. This can be done,for example with the aid of the apparatus shown in FIGURE 11, having anoutlet channel 114 in its uppermost portion, and an inlet channel 113 inits lowermost portion, as Well as a distribution grid 120 disposed inits lower portion. The regenerating unit 118 communicates with thereaction vessel 112 via an upper channel 117 and a lower channel 119.The regenerating unit 118 also has an inlet channel 116 and an outletchannel 115.

The reaction vessel 112 is supplied with the 4,4-dimethylm-etadioxaneand water steam through inlet channel 113 and the vapors leave thereaction vessel through outlet channel 114. A fluid catalyst circulatesoutside of the reaction vessel following the course 117, 118, 119,whereas a mobile catalyst circulates in the reversed direction that isfollowing 119, 118, 117. During this passage the catalyst is broughtinto contact with small quantities of phosphoric acid which areintroduced into the re-generating unit 118 through inlet channel 116.The phosphoric acid is mixed with inert gases or vapors and after havingcontacted the catalyst leaves through outlet channel 115.

The afore-mentioned apparatus are particularly useful for regeneratingthe catalyst by re-irnpregnation with phosphoric acid. However, they canbe easily adapted for use as regenerating units burning the impuritiesand deposits on the catalyst with such slight modifications well withinthe reach of any person skilled in the art.

Although the reimpregnation with phosphoric acid can be done separatelyit will be useful to have an apparatus wherein both regeneratingprocesses can be combined. This is shown in FIGURE 12, wherein thereaction vessel is combined with a calcining furnace 121, and, disposedwithin the furnace a reimpregnation unit 122. Within the reaction vesselthere is disposed an intermediate unit 140 having a distribution grid131 and an inlet channel 128. Furthermore, there are provided two inletchannels 132 and 13 3, and in the interior of the intermediate unit 140,injectors 136 and 137 at the end of channels 132 and 133 respectively.

Channel 134 leads into the calcining furnace 121 and channel 135 leadsinto the reimpregnation unit 122. At its lower end, the reaction vessel120 has an inlet channel 123 and a distribution grid 124. Within thereaction vessel 120 there is provided a cyclon with an outlet channel126. The reaction vessel is in communication with the reimpregnationunit 122 via the channel 143. The reimpregnation unit 122 has an inletchannel 141, a distribution grid 142, and a channel 144 connected to thecyclon 145 in the calcining furnace 121, the cyclon being connected withan outlet channel 146'. The calcining furnace 121 has an inlet channel138 and a distribution grid 139.

The reaction vessel 120 is fed with a mixture of 4.4-dimethylmetadioxane and water steam through channel 123 and distributedby grid 124. The gas stream passes through cyclon 125 and leaves thereaction vessel through channel 126 as indicated by arrow 127. A smallportion of the catalyst contained in the reaction vessel 120 passescontinuously to the intermediate Zone (stripping zone) 140 where it ismade fluid by the steam arriving through channel 128, as indicated byarrow 130 and distributed by grid 131. The steam takes along the tracesof the 4.4- dimethylmetadioxane carried by the catalyst and also leavesthrough channel 126. The catalyst then passes through channel 134 intothe calcining furnace 121. It is moved by a stream of air coming fromchannel 132 as indicated by arrow 132a, introduced in predetermineddosages by injector 136. In the calcining furnace 121 the catalyst ismade fluid and burns with a stream of air being fed thereinto throughchannel 138 as indicated by arrow 138a and distributed by the annularsection of grid 139. The catalyst remains for sometime in the calciningfurnace 121 and then falls back into the reimpregnation unit 122. Aportion of the catalyst in the intermediate Zone 140 can be directly fedinto the reimpregnation unit 122. This is done through channel 135 bymeans of water steam which latter is supplied through channel 133 inpredetermined dosages by injector 137. This steam contains phosphoricacid supplied through channel 133b. In the reimpregnation unit 122 thecatalyst is made fluid by water steam supplied through channel 141 asindicated by arrow 141a and distributed through grid 142. In thereimpregnation unit the reimpregnation is continued and completed whichhad already started in channel 135. The catalyst which is thusreimpregnated with phosphoric acid returns into the reaction vessel byits proper gravity and through the channel 143 as indicated by arrow143a. The gases and vapors leave the reimpregnation and calcining zonesby passing through channel 144, the cyclon 145 to the outside through anoutlet 146 (see arrow 147).

The circulation of the catalyst is controlled by the particulardimensions of injectors 136 and 137. By interrupting circulationsthrough channel 135 the entire catalyst is first calcinated before beingreimpregnated. If the rate of flow through channels 134 and 135 is equalit then follows statistically that the catalyst will be reimpregnatedtwice before undergoing a calcination. By increasing the rate of flowthrough channel 135 without modifying the rate of flow through channel134 this ratio can be further modified, so that a reimpregnation iseffected more than twice for each calcination.

The afore-described apparatus offers great advantages Example I 100grams of quartz composed of grains having a diameter of 0.6 to 0.9millimeter and a specific surface of 3 cmF/grarn are immersed in anaqueous solution of phosphoric acid containing 40% by weight of acid.They are then dried in the open air until there remain only 5.58 gramsof solution absorbed at the surface of the grains. The catalyst is thendried in a dryer at a temperature of 280 'C. for about 10 hours. Afterdrying, the total weight of the catalyst is 102.20 grams. Thiscorresponds to a phosphoric acid content of the catalyst of 2.15%.

Example II Example I is repeated with an aqueous solution of phosphoricacid containing 10% by weight of acid. The final weight of the driedcatalyst is 100.3 g., corresponding to a phosphoric acid content of0.3%.

Example 111 Example I is repeated with an aqueous solution of phosphoricacid containing 70% by weight of acid. The final Weight of the catalystis 105.2 g. corresponding to a phosphoric acid content of 5% of thedried catalyst.

Example IV 4.4-dimethylmetadioxane and water are injected together intoa vaporizer-preheater each at a constant rate of 0.6 liter per hour. Themixture of vapors is introduced into a reaction vessel, heated to 280 C.and passed through a catalytic bed of 1.5 kilograms of quartzimpregnated with phosphoric acid, as described in Example I. Thecatalyst has a volume of 1.15 liters. The spatial speed of the vapors,with respect to each of the corresponding injected liquids, is 0.52liter/hour/ liter of catalyst.

The vapors obtained from the reaction vessel are condensed and thenneutralized by adding sodium hydroxide. They are then fractionated bydistillation. The non-converted 4.4-dimethylmetadioxane is reenteredinto the reaction vessel.

After seven days 45.6 kilograms of 4.4 dirnethylmetadioxane have beenconsumed. As a yield there are obtained 24.3 kilograms of hydrocarbonsand 11.21 kilograms of formaldehyde in an aqueous solution, as well as3.33 kilograms of high-molecular products containing primarily3-methylbutane-1.3-diol.

The chromatographic analysis of a cut of said hydr carbons shows thatthey are composed of isoprene and a small quantity of isobutene. Byfractional distillation 23.9 kilograms of isoprene and 0.4 kilogram ofisobutene are obtained.

The activity of the catalyst has only slightly decreased, its weight nowbeing 292 grams and its final activity being 70% of its initialactivity. The ratio of the final and the initial rates of conversion arethus 70:100.

The average rate of conversion is 45.2%.

The following molar yields are obtained from the above values:

Percent Isoprene 89.5 Isobutene 1.8

actual yield of formaldehyde theoretical yield of formaldehyde In thisformula T represents the rate of conversion of 4.4-dimethylmetadioxaneinto formaldehyde, i.e. the molecular ratio between the reacting4.4-dimethylmetadioxane resulting in the production of formaldehyde, andthe converted 4.4-dimethylmetadioxane, which is equal to T isobuteneisoprene converted 4.4-dimethylmetadioxane isobutene +isoprene converted4.4-dimethylmetadioxane obtained formaldehyde (2 isobutene +isoprene) Inthis example P is equal to 93.2%.

Example V 4.4-dimethylmetadioxane and water are injected together into avaporizer-preheater each at a constant rate of 0.12 liter per hour. Themixture of vapors is introduced into the apparatus, as described inFIGURE 9, and passed at 275 C. through a fluid catalytic bed of grams ofgrains of sand having an average diameter of 200 microns, a specificsurface in the order of 50 cm. gram, and having been impregnated withphosphoric acid according to the method described in Example I, the acidcontent being 1%. The apparent density of the catalyst is 1.55, and thespatial speed of the vapors is, with respect to each of the injectedliquids, 1.55 liters/hour/ liter of catalyst. The average residence timeof the catalyst in the reaction vessel is of about 3 hours.

The vapors obtained from the reaction vessel are condensed andneutralized by adding sodium hydroxide. They are then fractionated bydistillation. The non-com verted 4.4-dimethylrnetadioxane is reenteredinto the reaction vessel.

After 17 hours, 1,420 grams of 4.4-dimethylmetadioxane have beenconsumed. As a yield there are obtained 716 grams of isoprene, 20 gramsof isobutene, 102 grams of secondary products and 336 grams offormaldehyde. The deposits on the catalyst amount to 27 grams.

The rate of conversion of the 4.4-dimethylmetadioxane is 71%.

The yield of formaldehyde (P is thus 91.4%, the yield of isoprene 86%,and the yield of isobutene only 2.9%.

Example VI Example IV is repeated with the following modification:

The 4.4-dimethylmetadioxane is injected into a vaporizer together withcyclohexane instead of water, and each at a constant rate of 0.06liter/hour. The temperature in the reaction vessel is maintained at 270C., the catalyst forms a fixed bed composed of grams of the sameimpregnated quartz, as described in Example I. The volume occupied bythe catalyst is 116 cm}, and the spatial speed is 0.52 liter/hour/liter.

The yield after 52 hours with respect to a consumption of 1,380 grams of4.4-dimethylmetadioxane is as follows: 687 grams of isoprene, 22 gramsof isobutene, 326 grams of formaldehyde, 120 grams of secondaryproducts, 28 grams of deposits on the catalyst.

The average rate of conversion is 46%, the yield of isoprene 85%, theyield of isobutene 3.3%, and the yield of formaldehyde (P 91%.

Example VII Example IV is repeated with 1.5 kg. of the catalyst asdescribed in Example II.

The yield after 52 hours with respect to a consumption of 5.4 kg. of4.4-dimethy1metadioxane is as follows: 2.82 kg. of isoprene, 60 g. ofisobutene, 1.27 kg. of formaldehyde, 360 g. of secondary products and 30g. of deposits on the catalyst.

The average rate of conversion is 18%, the yield of isoprene 89% and theyield of formaldehyde (P 91%.

Example VIII Example IV is repeated with 1.5 kg. of the catalyst, asdescribed in Example III.

The yield after 43 hours with respect to a consumption of 17 kg. of4.4-dimethylmetadixane is as follows: 8.5 kg. of isoprene, 130 g. ofisobutene, 3.8 kg. of form-aldehyde, 900 kg. of secondary products and425 g. of deposits on the catalyst.

The average rate of conversion is 68%, the yield o isoprene 85% and theyield of formaldehyde 86% Example IX Example IV is repeated at atemperature of 200 C.

The yields are the same as in Example IV, but the conversion rate isonly 21%, the deposits being slightly increased.

What we claim is:

1. A process for producing isoprene and formaldehyde comprising the stepof passing, at a temperature of from 200 to 300 C. a mixture of4.4-dimethylmetadioxane with an inert diluent in the vapor phase througha catalyst consisting of silica having a specific surface not exceeding100 mP/gram, impregnated with phosphoric acid to such an extent that theacid content of the catalyst is kept Within the range of from 0.3 to byWeight.

2. A process for producing isoprene and formaldehyde comprising the stepof passing, at a temperature of from 200 to 300 C. a mixture of4.4-dimethylmetadioxane with an inert diluent in the vapor phase througha catalyst consisting of quartz impregnated with phosphoric acid to suchan extent that the acid content of the catalyst is kept within the rangeof from 0.3 to 5% by Weight.

3. A process for producing isoprene and formaldehyde comprising the stepof passing, at a temperature of from 200 to 300 C. a mixture of4.4-dimethylmetadioxane with an inert diluent in the vapor phase througha catalyst consisting of silicious sand having a specific surface notexceeding 100 m. gram, impregnated with phosphoric acid to such anextent that the acid content of the catalyst is kept within the range offrom 0.3 to 5% by weight.

4. A process for producing isoprene and formaldehyde comprising the stepof passing, at a temperature of from 200 to 300 C. a mixture of4.4-dimethylmetadioxane with an inert diluent in the vapor phase througha catalyst consisting of silicious sandstone having a specific surfacenot exceeding m. gram, impregnated with phosphoric acid to such anextent that the acid content of the catalyst is kept within the range offrom 0.3 to 5% by weight.

5. A process for producing isoprene and formaldehyde comprising passing,at a temperature of from 200 to 300 C. a mixture of4.4-dimethylmetadioxane with an inert diluent in the vapor phase througha catalyst consisting of catalytic particles of silica having a specificsurface not exceeding 100 m. gram, impregnated with phosphoric acid tosuch an extent that the acid content of the catalyst is kept Within therange of from 0.3 to 5% by weight, and periodically displacing saidparticles relative to one another.

6. A process for producing isoprene and formaldehyde comprising thesimultaneous steps of continuously passing, at a temperature of from 200to 300 C. a mixture of 4,4-dimethylmetadioxane with an inert diluent inthe vapor phase through catalytic mass of particles consisting of silicahaving a specific surface not exceeding 100 m. gram, impregnated withphosphoric acid to such an extent that the acid content of the catalystis kept within the range of from 0.3 to 5% by weight, and contained in areaction vessel displacing said particles relative to one anotherwithdrawing a portion of said catalytic mass from the reaction vessel,re-impregnating the same in a zone outside from said reaction vessel,and recycling it to the latter.

7. A process for producing isoprene and formaldehyde comprising the stepof passing, at a temperature of from 200 to 300 C., a mixture of4.4-dimethylrnetadioxane with an inert diluent in the vapor phase,through a catalyst consisting of silica having a specific surface notexceeding 100 m. gram impregnated with phosphoric acid to such an extentthat the acid content of the catalyst is kept within the range of from0.3 to 5% by weight, at a spatial speed of the liquid4.4-dimethylmetadioxane of from 0.2 to 3 liters per hour and per literof catalyst.

References Cited in the file of this patent UNITED STATES PATENTS1,841,055 Reepe et a1 Jan. 12, 1932 2,218,640 Friedrichsen et al Oct.22, 1940 2,241,777 Friedrichsen May 13, 1941 2,361,539 Friedrichsen Oct.31, 1941 FOREIGN PATENTS 468,654 Italy Jan. 29, 1952

1. A PROCESS FOR PRODUCING ISOPRENE AND FORMALDEHYDE COMPRISING THE STEPOF PASSING, AT A TEMPERATURE OF FROM 200 TO 300*C. A MIXTURE OF4,4-DIMETHYLMETADIOXANE WITH AN INERT DILUENT IN THE VAPOR PHASE THROUGHA CATALYST CONSISTING OF SILICA HAVING A SPECIFIC SURFACE NOT EXCEEDING100M.2/GRAM, IMPREGNATED WITH PHOSPHORIC ACID TO SUCH AN EXTENT THAT THEACID CONTENT OF THE CATALYST IS KEPT WITHIN THE RANGE OF FROM 0.3 TO 5%BY WEIGHT.